The formation mechanisms and chemical compositions of brown dwarfs likely span a diverse range .
If they are formed predominantly in isolation by gravitational collapse like dwarf stars then their compositions might follow those of dwarf stars , spanning a relatively narrow range in metallicities ( \sim -0.4 to +0.4 dex ) and predominantly oxygen-rich ( C/O \lesssim 0.7 ) .
On the other hand , giant planets in the solar system are all super-solar in metallicity ( [ C/H ] > 0.5Â dex ) , which is thought to be a consequence of formation by core-accretion in a circumstellar disk .
In this study we determine the atmospheric chemical compositions of three brown dwarfs and find them to be neither characteristic of dwarf stars nor giant planets .
We derive high-precision atmospheric chemical abundances using high-S/N HST near-infrared spectra of three mid T Dwarfs which , together with two previously studied objects , display distinctly sub-solar metallicities and span C/O ratios of 0.4-1.0 .
These abundance patterns indicate either an old sub-stellar population and a multitude of formation environments or objects formed in circumstellar disks followed by ejection later .
We show that for a simple synthetic model population of brown dwarfs with the same spectral-type as our sample the predicted age distribution is predominantly older than the Sun and , hence , more metal poor , consistent with our derived abundances .
The diverse C/O ratios we find are indicative of either different chemical reservoirs in their formation environments or different formation pathways .
Our results open the possibility that cool brown dwarfs may provide important probes of early galactic chemical evolution and inhomogeneity .